This invention relates to a system for voice and data communication, and more particularly to a system for satellite communication in rural, remote, or sparsely populated areas, in areas lacking a conventional telephone infrastructure, and in areas such as airports and convention centers that are frequented by international travelers. These areas can be called generally "local areas".
Modern telephone networks have evolved mainly to address the need for service in densely populated, highly developed metropolitan areas. Thus, in many less developed countries where there is no concentrated need for service, no communication infrastructure exists to support telephone service. In developed countries, telephone service is typically provided under a governmental mandate that requires universal service for metropolitan and rural subscribers. Because today's telephone networks are optimized for densely populated areas, the mandated universal telephone service is inherently uneconomic when used in rural, remote, or sparsely populated areas. Several attempts have been made to solve these problems.
One possible solution is the use of microwave links to extend the reach of conventional telephone technology employed in metropolitan areas to rural, remote, or sparsely populated areas. This approach is inadequate for less developed countries that have no communication infrastructures to support the microwave links. Even for a country that has a communication infrastructure, maintenance is prohibitively expensive with this approach because the equipment required to extend the existing telephone technology to rural areas is remotely located, sometimes in unpopulated areas, and therefore is exposed to vandalism and natural hazards. The maintenance expense is further increased by the long travel-times needed for service personnel to reach the remotely, possibly inconveniently, located equipment. Moreover, the small subscriber pool in a rural area makes it uneconomic to extend a wireline network to such an area.
Another possible solution is the use of high-power land-mobile-radio transmitters, such as those used for police, fire, and other emergency services, to provide local area coverage. The general lack of a well developed communication infrastructure for connecting a land-mobile-radio system to the public switched telephone network (PSTN) is a serious drawback of this solution. Further, to provide sufficient geographic coverage, many transmitter sites would have to be deployed.
A third possible solution is the use of a cellular radio system. Like wireline telephone networks, however, cellular systems have evolved to address the needs of highly developed, densely populated areas. Thus, as with wireline networks, less developed countries lack the communication infrastructures for supporting cellular service. In developed countries, the cell sizes in cellular systems have steadily decreased in order to maximize economic efficiency in high-use, densely populated urban environments. In contrast, the adaptation of cellular radio systems for rural, remote, or sparsely populated areas would require large cells; thus, cellular radio systems have generally been recognized as less attractive alternatives to satellite communication systems if the latter are available.
A variant of the cellular radio solution is a multi-line terminal that permits multiple subscribers to gain access to a cellular system using conventional wireline telephones. This approach is explained in detail in U.S. patent application Ser. No. 08/505,665 to Henry et al., entitled "Analog Fax and Modem Requests in a D-AMPS Multi-Line Terminal System", filed on Jul. 21, 1995, which is expressly incorporated here by reference. While this approach does not require subscribers to replace their analog wireline phones with digital cellular phones, it does not remove the inefficiencies noted above in applying cellular radio technology to rural areas.
FIG. 1A illustrates another possible solution that employs a satellite communication system. In FIG. 1A, one of a plurality of subscriber units 100 communicates with a satellite system 200 via an antenna 150. The satellite system 200 comprises at least a single geostationary satellite or a plurality of non-geostationary satellites. The satellite system 200 may operate, for example, in the manner of today's Inmarsat or in the manner proposed for the Iridium, Odyssey, or ACES systems. The satellite system 200 also communicates with an earth station 400 via an antenna 450. The earth station 400 transmits traffic from the satellite to a public network, such as a PSTN, or from the network to the satellite via a gateway switch 500.
Various aspects of conventional earth stations and satellite systems are described in the literature, including David Calcutt and Laurie Tetley, Satellite Communications: Principles and Applications, chapters 8 and 9 (1994); and U.S. Pat. Nos. 4,901,307 to Gilhousen et al.; U.S. Pat. No. 5,446,756 to Mallinckrodt; and U.S. Pat. No. 5,455,823 to Noreen et al. Information is transmitted according to various modulation and access schemes, including amplitude, frequency, and phase modulations, and frequency division, time division, and code division multiple access.
A significant disadvantage of the conventional satellite approach is that it imposes a high demand on the transmission capacity of the satellite transponders because the subscribers' local traffic and signaling are routed through the satellite. "Local traffic" is traffic between subscriber units 100. Consequently, this approach is primarily used to supplement existing wireline or cellular systems, rather than to provide the basic communication infrastructure. In practice, this approach is typically limited to supporting roughly 1,000-to-20,000 subscribers per satellite, simultaneously, in contrast to a conventional central office switch or a private branch exchange (PBX) that can simultaneously support in excess of 100,000 subscribers.
Other disadvantages of the conventional satellite approach arise from the constraints under which satellite systems operate. These constraints include the limited electrical power available aboard a satellite and the limited radio bandwidth used for interference-free communication with the earth station. Of course it will be appreciated that even some terrestrially based radio signal repeaters suffer the same constraints, which can arise from an inaccessible and remote location of the repeater.
FIG. 1B illustrates a variant of the conventional satellite communication system depicted in FIG. 1A. In this variant, PBXs 510, 520 are used with wideband, high-capacity satellite earth stations 410, 420, respectively. In such a system, PBXs 510, 520 internally switch local traffic between the subscriber units 100 to which they are connected and rely on the satellite system 200 for trunking between them. This variant suffers from the disadvantage of requiring either expensive, complex earth stations 410, 420 or complex and expensive subscriber units 100 for communicating with the satellites. Also, such systems are typically optimized for densely populated environments, particularly large commercial office parks or large private corporate networks. Because wideband, high-capacity earth stations are required, the cost and the attention required of an operator make this approach unattractive for use in rural, remote, or sparsely populated areas or for special-purpose applications such as airport coverage.
The communication system described in the patent to Gilhousen et al. cited above is an example of this variant. The patent describes a system having one or more satellite or terrestrial repeater stations and one or more central stations that communicate information among mobile or fixed, local or remote users. The satellite repeater stations can communicate directly with the users or indirectly through a central station and the terrestrial repeaters to the users. The system preferentially uses the satellite repeaters to interconnect users, especially rural users, although as users come within range of terrestrial repeaters, those users can be linked through the terrestrial repeaters. The patent also mentions that the users can be linked directly.
Thus, there is a need for a communication system that can efficiently provide service to subscribers in urban environments with poor telecommunication infrastructure, in rural, remote, or sparsely populated areas, and in special areas, such as airports and other transportation hubs. Such a communication system would ideally be adaptable to a wide variety of circumstances and population densities, thereby enabling it to serve efficiently both rural and urban needs.